Each year in the United States there is an increasing trend of patients receiving total joint replacement (TJR) procedures. As reported by the National Center for Health Statistics in 2004, total hip replacements (THRs) have risen to 234,000 per year and total knee replacements (TKRs) have risen to 478,000. This is in part due to revision surgeries, but also due to patients receiving TKRs at younger ages because of higher consumer demand and improved surgical outcomes. Additionally, surgeons have begun allowing patients in their 80s and later, even with co-morbid conditions, to receive TKR implants in order to increase quality of life, thus also increasing the total number of implant procedures.
Unfortunately, joint longevity is an increasing problem, particularly in light of the younger ages of patients. As the age of patients' receiving arthroplasty decreases, there is increasing interest in longevity of the implant life to reduce the number of potential revisions a patient must undergo. Failure rates vary drastically due to, e.g., design; cemented, hybrid, or non-cemented implant; and age at implantation. Similarly, failure can be caused by a variety of factors including, e.g., infection, stress shielding, pen-prosthetic osteonecrosis or fracture, implant instability, polyethylene wear-through or fracture, allergy or hypersensitivity. The most common cause of failure of an implant is implant loosening resultant of wear and osteolysis.
The use of ultra high molecular weight polyethylene (UHMWPE) inserts in total knee replacements, though relatively inert, result in wear particle-caused osteolysis, the predominant cause for prosthesis failure and revision surgery. The release of polymer particles due to wear of the implant surface leads to macrophage activation which subsequently activates osteoclasts via cytokine release. Osteoclasts cause bone resorption and loosening of the implant, and the cycle continues until failure. Even with the highest performing polymer found to date for joint replacement, ultra-high molecular weight polyethylene (UHMWPE), particle formation has been found to exist irrelevant of the counterpart bearing surface.
In an attempt to alleviate this problem, bisphosphonates have been utilized via oral systemic delivery. While bisphosphonates can decrease periprosthetic osteolysis, this method requires the patient to periodically ingest the agents for the life of the implant. Potential problems with such delivery routes include the need for the patient to remember to take the medication as well as the need for the patient to have long-term access to the medication. In addition, the systemic delivery and the high solubility of bisphosphonates result in a predominant portion being excreted via the kidney without reaching its target as well as the possibility of unintended consequences due to the circulation of the bisphosphonates throughout the patient's system.
What are needed in the art are UHMWPE composite materials that can alleviate problems found in joint replacements, for instance problems associated with the release of particles from the materials due to wear. What are also needed are composite materials that can take advantage of the existence of wear particles formed during use of the implant.